Epoxy resin based protective coating composition for printed circuit boards

ABSTRACT

A resinous protective coating useful in adhering plastics to metallic substrates. The protective coating is particularly useful as a solder mask and in the manufacture of printed circuit boards with circuit patterns having a resolution of at least 0.25 mm lines and spaces thereon. The protective coating comprises resins dissolved in solvents, wherein the coating, upon being subjected to heat, goes from a liquid or soft &#34;gel-like&#34; state into a solid phase without bleeding. The protective coating is resistant to copper, nickel and gold electroplating baths, adheres to insulating substrates, adhesive coated base materials as well as metallic substrates and is capable of withstanding the thermal shock of dip soldering.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 103,600,filed Dec. 14, 1979, now U.S. Pat. No. 4,510,276, which is acontinuation-in-part of U.S. application Ser. No. 103,340, filed Dec.13, 1979, now abandoned, which in turn is a continuation-in-part of U.S.application Ser. No. 3,099, filed Jan. 12, 1979, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to a resinous protective coatingcapable of being applied to preselected portions of an insulating ormetallic substrate. More particularly, the present invention relates toa resinous composition that adheres well to metallic and plasticsurfaces and adheres especially well to copper, a surface which isdifficult to bond to. The present invention also relates to a resinousprotective coating for use in producing patterns on circuit boards, theprotective coating including a highly functional solid resin having amelting point between about 60° C. and about 200° C.

2. Description of the Prior Art

A problem in the prior art is the poor adhesion of plastics to coppersubstrates. It is well known, for example, that copper oxidizes evenwhen covered with a coating layer and that consequently organic coatingsdo not adhere as well to the loosely adherent oxidized copper films. Infact, the organic coating separates after a period of time from thecopper substrate due to postulated diffusion of oxygen through thecoating on the copper substrate and subsequent oxidations of the coppersubstrate. Attempts to solve this problem have included precoating thecopper substrate with an adherent copper oxide, e.g., a hot alkalinehypochlorite solution such as Ebonol C (commercially available fromEnthone Company, a division of Asarco, West Haven, Conn.), or coatingthe copper substrate with brass or zinc plating.

Also, methods employed in the manufacture of printed circuit boardstypically contain at least one step to which a permanent protectivecoating is employed. Illustrative of these methods are the following:

In one method of producing circuit boards, a background pattern isprinted with a permanent protective coating which leaves exposed apattern on an adhesive coated laminate. The adhesive coated laminatecontains catalytic material therewithin which allows the laminate toinitiate electroless metal plating. The permanent protective coating ormask is hardened by curing, and the exposed pattern on the laminate isactivated in an oxidizing solution, such as chromic-sulfuric acidsolution to render the exposed pattern receptive to electroless copperplating with good adhesion of the copper to the activated surface. Thepermanent mask resists activation by the chromic-sulfuric acid solution.Copper is deposited on the exposed areas of the adhesive coated laminateto form an electrically conductive pattern via electroless deposition.

In another method of producing circuit boards, a laminate is coated witha non-catalytic adhesive. The entire laminate is then etched with anoxidizing solution, such as a chromic-sulfuric acid solution beforebeing seeded and sensitized with a catalytic solution, such aspalladium-tin-chloride. A background permanent mask or protectivecoating is then printed on the laminate before copper is electrolesslydeposited on the exposed areas of the laminate in the form of theprinted pattern.

In another method of producing printed circuit boards, a "print andetch" technique is employed. Copper foil is laminated to one or moresides of an insulating substrate, a positive pattern is printed (screen,photographically, etc.) with a temporary protective coating or resist,background copper is then etched away by subjecting the substrate to anetching solution, e.g. ferric chloride, ammonium persulfate and thelike, the temporary resist is moved from the substrate using appropriatesolvents, e.g. alkali soluble resists are removed with alkali solvents,holes are produced on the substrate by a laser beam, punching, drillingand the like, the substrate is sensitized and seeded by contacting itwith, for example, aqueous acidic solutions of stannous tin ions andprecious metal ions such as palladium ions, a registered solder maskwhich is a permanent protective coating is printed (screen, etc.) ontothe substrate leaving lands and holes exposed, copper is electrolesslydeposited onto the exposed lands and holes, but not the heat curedsolder mask and the substrate is then dipped into molten solder to causesolder to adhere to the copper coated lands and holes.

In another method of producing printed circuit boards, a "fullyadditive" technique is employed. A suitable insulating substratum isprepared having a distance between hole centers of about 2.5 mm or less,the substratum and walls of the holes are sensitized and seeded usingknown seeding and sensitizing agents such as stannous chloride,palladium chloride activation, a permanent protective coating or resistis screened to produce a permanent background resist leaving the desiredcircuit pattern exposed, the pattern having as low as about 0.35 mmbetween lines, the resist is heat cured; copper is electrolesslydeposited on the exposed pattern and in the holes and the board is thendip soldered.

In another method of producing printed circuit boards, a "semi-additive"technique is employed. An insulating substrate is prepared having holeson 2.5 mm centers, the walls of the holes and the surface(s) of thesubstrate are sensitized and seeded in a conventional manner, copper iselectrolessly deposited over the entire surface of the board and in theholes, a temporary protective coating or resist is employed to print acircuit pattern having 0.35 mm lines, the temporary resist is heatcured, the circuit pattern is built up by electroplating a metal ontothe exposed areas of the substrate, the temporary resist is removed, thethin layer of electroless copper which had been covered by the mask isetched away, a permanent registered solder mask is printed and heatcured, and the substrate is then dip soldered.

In another method of producing circuit boards, a printed circuit isprepared on an insulating substratum by the print and etch technique, apermanent registered solder mask is printed thereover leaving lands andholes exposed, the permanent mask is cured, copper is electrolesslydeposited on the walls of through-holes formed in the insulatingsubstratum, and the circuit is dipped in molten solder to cause solderto adhere to the lands and holes. Another modification of these methodsinvolves using a fully additive process to prepare the printed circuitboard on the first step, the remaining steps being the same.

The permanent protective coatings used as resists, solder masks, etc. inthe prior arts methods of producing circuit boards have had a number ofdeficiencies which are set forth subsequently herein. For high densitycircuits, it has been customary to use dry film photoresists which areextremely costly. Screen printed resists or masks, while economical,have not been able to achieve the high resolution necessary for highdensity circuit boards. For example, the problems associated with theuse of prior art solder masks such as the PC-401 Series Solder Mask(commercially available from Kollmorgen Corporation, Glen Cove, N.Y.)are detailed hereinbelow as an illustration of the type of problem alsoassociated with the use of other resinous protective coatingcompositions of the prior art as permanent resists or the like.

Virtually all printed circuit board assemblies, even in small quantityproduction, are wave or dip soldered. Heretofore, in producing circuitboards which have a high circuit density per unit area, difficulty hasbeen experienced due to the fact that the holes in such boards: (1) tendto have an extremely small diameter e.g. 0.35-1 mm; and (2) tend to beextremely closely spaced at least in some portions of the circuitry.High density boards may also have hole centers spaced a distance of lessthan 1 mm. When the holes are less than 2.5 mm apart with a conductorbetween the holes, it is practically impossible to print a registeredpermanent solder mask over the conductor without smearing solder maskaround the lands adjacent the holes. In conventional practice, a platedthrough hole circuit board is produced with a circuit on one or moreexposed surfaces. Before soldering, a registered solder mask is printedon the circuit pattern(s) to leave holes and lands or pads (i.e., smallareas on the surface surrounding to holes) as well as fingers (i.e.,terminal or contact areas of the circuit pattern) exposed. Subsequently,the components are fastened to the circuit as by passing over a solderwave to apply solder to the component leads and on the exposed areas,i.e., on the exposed lands and in the metallized holes. The solder maskprotects the major portion of the circuit from the solder and thusguards against short circuiting by confining solder to only those areaswhere it is desired.

In such conventional circuits, the lands or pads are exposed whileconductor lines making up the conductor pattern or patterns areprotected by the solder mask. Accordingly, when the circuit density ishigh, it is extremely difficult to print a registered solder mask so asto provide exposed land or pad areas surrounding the holes without somesolder mask accidentally lodging on the barrel of the hole.

Conventional registered, screen printed solder masks have otherdisadvantages. To maintain fine printing tolerances in such boards,extremely thin prints are used. Thus, even when great precautions aretaken in printing the solder mask on high density circuit boards of thetype described, there is a good possibility of the masks breaking downin part, thereby causing the solder to bridge from one land to another,or from one conductor line to another, which in turn results in shortcircuiting on the finished board. When thicker prints are used to avoidmask breakdown, the solder mask tends to block the holes, therebypreventing proper soldering.

In the prior art, various thermosetting resinous compositions have beenused to provide protective coatings such as plating resists, temporaryor permanent resists and solder masks. The thermosetting resins used inthese compositions had low melting points were liquid at roomtemperature. These resinous protective coatings were incapable ofproviding lines and spaces having a resolution lower than 0.6 mm withoutbleeding over into the exposed holes and lands surrounding the holeswhen a conductive pattern was being screened. These resins flowed attemperatures from about room temperature up to about 160° F. duringcuring. Consequently, after the prior art thermosetting resinouscompositions had been screen printed onto an insulating substrate oronto a metallic pattern already on substrate, as the substrate washeated to evaporate the solvent in the resinous composition, theresinous composition would further liquify. The printed resinouscomposition pattern would become extremely liquid and "bleed" or spreadto cause poorly defined, fuzzy, pattern edges thereon. When suchbleeding or spreading occurred, solder would not coat pattern edgeswhich border lands and holes during the subsequent dip/wave solderoperation.

Only after "bleeding" or spreading out of the printed pattern of theresinous composition would the resinous composition become polymerizedand solidified (thermoset). This deficiency was attributable to the factthat in the curing step the resins would liquify before they hardened.During this liquification phase, the resinous composition further spreadout to create a poorly defined pattern edge which borders areas wheresoldering is desired. Liquification of the resins during cure resultedin sloppy mask edges, i.e., the original print was sharp, but duringcure the edges of the printed mask spread. Edges which are sharp andhave not spread are quite important in high density boards, but notoverly important in low density boards.

Prior art masking compositions were not very chemically resistant toadhesion promotion solutions such as chromic acid and highly alkaline,electroless copper baths used in the "fully additive" technique.Moreover, the prior art masking compositions were not smooth andproduced rough coatings whose surfaces would be receptive to theproduction of adherent extraneous copper. With their use in makingcircuitry using the additive technique, unwanted spots of copper wouldappear on a background resist pattern for the reasons given hereinabove.Surface resistivity between conductors would be lowered unless theextraneous copper was removed by an extra step such as brushing.

Another problem unique to printed circuit boards is that the hardenersor curing agents used in prior art solder mask compositions stain golddeposits such as exposed tabs or fingers.

SUMMARY OF THE INVENTION

1. Objects of the Invention

It will be seen from the above that there is a need for permanentprotective coating compositions for use in the manufacture of printedcircuit boards, therefore, which are resistant to copper electroplatingbaths, adhere to insulating substrates, adhesive coated base materials,as well as metallic substrates, do not bleed when cured, are capable ofwithstanding the thermal shock of dip soldering and which maintain theirgel-like structure over a long period of time when subjected totemperatures of about 160° C.

It is, therefore, an object of the present invention to providepermanent protective coating compositions for use in the manufacture ofprinted circuit boards.

An object of the invention to provide high resolution screen printableresists.

Another object of the invention is to provide an improved method ofimproving the resolution of conductive patterns on circuit boards.

An object of this invention is to provide an improved resist for screenprinting of patterns on circuit boards.

It is also an object of this invention to provide a solid resin basedprotective coating useful in producing printed circuit patterns on acircuit board the patterns including lines and spaces having aresolution of at least 0.25 mm and as low as 0.1 mm.

An object of this invention is to provide a thermoset resin basedprotective coating the solid resin having a melting point between about60° C. and about 200° C.

Another object of this invention is to provide a protective coatingcomprised of a blend of resins and solvents such that, during heat cure,the coating passes directly from a gel stage into a solid phase withoutliquifying.

Still another object of this invention is to provide a registered maskwhich, upon heat cure, retains its sharp edges and does not bleed overonto holes and lands surrounding the holes.

An object of this invention is to provide a thermoset resin basedprotective coating capable of adhering to adhesive coated base materialsas well as metallic substrates, particularly copper substrates, and ofwithstanding the thermal shock associated with dip soldering.

An object of this invention is to provide a smooth, glossy protectivecoating which is not conducive to the formation of adherent extraneouscopper in the additive method for producing circuit boards.

An object of this invention is to provide a protective coating whichmaintains an initial gel-like state and remains adherent to metallicsubstrates when heated for an extended period of time at a temperatureof about 160° C.

An object of this invention is to provide a method of improving adhesionof organic coatings to copper substrates.

An object of this invention is to provide a protective coating whichdoes not stain gold deposits such as exposed tabs or fingers on printedcircuit boards.

Additional objects and advantages of the invention will be set forth inpart in the description, or may be realized by practice of theinvention, the objects and advantages being realized and attained bymeans of the methods, processes, instrumentalities and combinationsparticularly pointed out in the appended claims.

2. Description of the Invention

The protective coating compositions described in my U.S. application,Ser. No. 3,099, filed Jan. 3, 1979 and in my U.S. application filed Dec.13, 1979, both entitled "HIGH RESOLUTION SCREEN PRINTABLE RESISTS",respond, in part, to the aforementioned needs. Disclosed in suchapplications are resinous protective coating compositions useful in themanufacture of printed circuit boards having circuit patterns with aresolution of at least 0.25 mm lines and spaces thereon. The protectivecoating compositions each comprised a highly functional solid resindisclosed in a solvent, the resin having a melting point between about60° C. and about 200° C. Each of the protective coating compositionsinitially constituted a liquid or soft gel, and when cured became asolid without bleeding. The present invention is considered to be animprovement on the invention described in my aforementioned U.S.applications, and its main features and objectives are thus similar tothose of such aforementioned U.S. applications. It has been found thatthe protective coating composition of the present invention providesimproved adhesion to metallic substrates and preserves its gel structurewhen heated over an extended period of time at a temperature of about160° C., as described more fully hereinafter.

The present invention provides an improved means of bonding plastics tocopper substrates, improved high resolution permanent screen printable,protective coatings, improved methods of producing printed circuitboards employing the improved protective coatings, and the improvedcircuit boards formed thereby. As will be clear from the followingdescription, there is used in the manufacture of circuit boards of thisinvention certain resinous protective coating compositions which do notbleed when cured.

The protective coating composition of the present invention providesimproved permanent coating or masks which are not subsequently removed.The additive technique and molten solder application steps employed inthe manufacture of printed circuit boards each involve the applicationof such a permanent protective coating or mask. Various methods ofproducing printed circuit boards such as those described previouslyherein include the above-described steps and/or techniques and/orcombinations thereof.

For illustrative purposes only and not intending the invention to be solimited, the application of the permanent protective coating compositionof this invention is subsequently described herein in conjunction with atypical method (employing the additive technique) in the manufacture ofprinted circuit boards. The surface of an insulating substrate on whicha printed circuit pattern is to be formed is coated with a permanentresinous protective coating or mask according to this invention. Themask has the characteristics of being able to resist attack by acids andalkali to which printed circuit boards will ordinarily be subjected inprocessing. The mask has a smooth, glossy surface. Holes definingcross-overs have been preformed or may then be formed in the insulatingsubstrate by any method which does not damage the mask or substratematerials surrounding the holes, such as by drilling, piercing orpunching methods. A catalytic insulating blank such as the one describedin U.S. Pat. Nos. 3,600,330 and 3,672,986 may be used as the insulatingsubstrate. The board is contacted with a strong oxidizing solution suchas chromic-sulfuric acid solution to adhesion promote the exposed orunmasked areas of the blank. Then the board is contacted with anelectroless metal deposition solution to metallize the walls of theholes and the conductor pattern. In the next step, the insulating blankhaving circuit patterns electrolessly deposited thereon is printed witha permanent solder mask according to the present invention leaving thehole walls and if desired, lands surrounding the holes as well as fingerareas, exposed. The permanent solder mask compositions of this inventionare capable upon curing, of forming a smooth, glossy surface which notonly is resistant to the thermal shock of soldering but also isresistant to flux removing solvents. The permanent solder maskcompositions of this invention adhere well to the metallic substrate(circuit pattern) even when subjected to temperatures of about 160° C.for about 16 hours.

A thermosetting resin in the protective coating having a high meltingpoint and a high functionality permits the solvent in the protectivecoating to evaporate and the curing operation to occur without meltingand liquifying the resin in the coating, thus avoiding spreading of thecoating composition during the curing operation. It is believed that theviscosity of the protective coating composition increases as thethermosetting resin therein cures into a solid. The high functionalityof the thermosetting resin also creates rapid cross-linking so that theprinted mask pattern does not bleed or spread between lines or onto thelands and holes as maximum curing temperatures of the mask are reached.Consequently, the mask retains sharp edges.

It has been discovered that a thermosetting resin based screening maskcapable of: (1) producing printed circuit patterns having a resolutionof at least 0.25 mm lines and spaces on a substrate; (2) withstanding acopper plating bath; (3) adhering to adhesive coated base materials aswell as to metallic substrates during extended heat treatment; and (4)withstanding the thermal shock of soldering and subsequent flux removal,is provided by a protective coating composition which comprises a highlyfunctional thermosetting epoxy resin dissolved in a solvent and apreselected aliphatic hardener as the primary curing agent. The primarycuring agent is corrosive to metals such as copper. The thermosettingepoxy resin is a solid at room temperature and does not melt attemperatures up to its curing temperature, typically between about 60°C. and about 200° C.

A simple test was devised to determine whether a particular solidthermosetting resin meets the criteria of the present invention. A chunkor sample piece of the thermosetting resin approximately 5-10 grams inweight is placed in an oven and heated to a temperature between 100° and160° C. The resin sample is then removed from the oven and visuallyexamined for indication of its melting or spreading. If the samplethermosetting resin melts or spreads during such heating test, it is notsuitable for the present invention. On the other hand, if thethermosetting resin becomes and remains tacky during such heating test,it is suitable for the present invention.

The thermosetting epoxy resin of the present invention preferablycomprises about 35-75% of the dry mask, the balance being curing agentsand modifier(s). The highly functional thermosetting epoxy resincomprises more than about 10 weight %, preferably more than about 12weight % of the protective coating composition of this invention. Thehighly functional thermosetting epoxy resin also comprises less thanabout 80 weight %, preferably less than about 60 weight % of theprotective coating composition of this invention.

By highly functional thermosetting epoxy resins are meant thosethermosetting epoxy resins having more than about 3 and less than about10 functional groups, preferably between about 4 and about 7 functionalgroups. Suitable thermosetting resins are epoxy resins, and blends ofepoxy resins having an average epoxide functionality between about 3 andabout 6 and a melting point between about 60° C. and about 200° C.

Suitable epoxy resins include functional epoxy Novolac or bisphenol Atype epoxy resins that exist normally as a solid at room temperature(having molecular weights between about 350 and 15,000) and have amelting point between about 60° C. and 200° C. For illustrativepurposes, the subsequent discussion of the thermosetting resin basedprotective coatings of this invention will be directed to epoxy resinbased protective coatings, but this is not intended to limit the scopeof the invention thereto.

Preferred epoxy resins have an average epoxide functionality above 3 andinclude the following commercially available epoxy resins listed inTable 1 hereinbelow:

                  TABLE 1                                                         ______________________________________                                                  Average                                                                       Epoxide          Durran                                                       Function-        Softening                                          Epoxy Resin                                                                             ality    EEW*    Point   Manufacturer                               ______________________________________                                        EPON 1031 4        220     178° F.                                                                        Shell Chemical                                                                Co.                                        ECN 1063  4        200     178° F.                                                                        Ciba Geigy                                                                    Corp.                                      ENC 1273  3.8      225     73° C.                                                                         Ciba Geigy                                                                    Corp.                                      ECN 1280  4.1      230     70° -80° C.                                                             Ciba Geigy                                                                    Corp.                                      ECN 1299  4.4      235     99° C.                                                                         Ciba Geigy                                                                    Corp.                                      DEN 439   3.8      200     58° C.                                                                         Dow Chem.                                                                     Co.                                        ______________________________________                                         *Average Epoxy Equivalent Weight                                         

Other solid epoxy resins which are believed to be highly functionalthermosetting resins within the scope of this invention include APOGEN1013 (commercially available from Apogee Chemical Co.); EPI-REZ 521having an average epoxy equivalent weight of 200 and a Durran softeningpoint between 70° C. and 80° C. (commercially available from CelaneseCorp.); EPI-REZ 5291 having an average epoxy equivalent weight of 310and a Durran softening point between 90° C. and 95° C. (alsocommercially available from Celanese Corp.); and EPOTURF 37-171 havingan average epoxy equivalent weight of 150 and a Durran softening pointbetween 172° F. and 180° F. (commercially available from ReichholdChemical Co.).

Other less functional epoxy resins may be optionally included in thecoating composition to reduce the brittleness of the cured coatingcomposition. Such epoxy resins include copolymers of epichlorohydrin(1-chloro-2,3-epoxy propane) which have melting points within the rangeof 70° C. to 191° C. and molecular weights of about 350 to 15,000.Although epichlorohydrin is the source of most important organicepoxides employed in the formation of the epoxy resins, other epoxidessuch as, for example, 1,2,3,4-diepoxy butane may be used. Moreover,while it is preferred to use epichlorohydrin in the preparation of theresinous polymeric epoxide of the present invention, otherepihalohydrins such as epibromohydrin also may be used advantageously.Similarly, epoxy resins derived from phenols other than bisphenol A maybe used including, for example, the reaction product of epichlorohydrinwith resorcinol, with phenols derived from cashew nut oils, withhydroquinone, with 1,5-dihydroxy napthalene or with2,2,5-tetrabis-(4-hydroxy phenyl)hexane. Phenolic intermediates of theresol type, hydrazines and sulfonamides, such as, for example,2,4-toluene disulfonamide may also be used for reaction with an organicepoxide to product epoxy resins suitable for use. Aliphatic epoxy resinsare suitable, including, for example, the reaction product ofepichlorohydrin with glycerol, with ethylene glycol or withpentaerythritol.

The less functional modifier epoxy resins comprise up to about 30 weight%, preferably up to about 15 weight %, and when used typically comprisemore than about 10 weight % of the protective coating composition ofthis invention.

The highly functional epoxy resin is dissolved with a suitable solventto prepare a solution between about 50% and 90% by weight highlyfunctional epoxy resin, and preferably about 70% by weight highlyfunctional epoxy resin. Suitable solvents include glycol ethers andesters such as diethylene glycol ethyl ether, ethylene glycol ethylether, ethylene glycol methyl esters, acetates of the glycol ethers,secondary butyl acetate, normal butyl acetate, primary amyl acetate andthe like.

The preselected primary curing agent or hardener suitable for the highlyfunctional epoxy resin in the protective coating composition is analicyclic amine which is corrosive to copper surfaces. The corrosivenature of such an amine to copper surfaces has in the past suggestedthat its use be avoided. Contrary to prior teachings, applicant hasdiscovered that excellent adhesion of the protective coatingcompositions of this invention to copper substrates may be achieved withthe use of alicyclic polyamine curing agents which are ordinarilyconsidered corrosive to copper substrates.

Preferred alicyclic polyamine curing agents include methane diamine;1,3-diamino-cyclohexane; isophorone diamine; triethylenediamine; andhexamethylenetetramine. The use of these preselected alicyclic polyaminecuring agents in the protective coating compositions of this inventionimproved their heat resistance and adhesion to metallic surfaces.

Gelling agents may be added without impairing properties of the coatingcomposition to improve antisag properties and screen printingtechniques. When racking, storing and/or curing of the protectivecoating on the substrate in a substantially vertical orientation iscontemplated, without the addition of the thickening or gelling agent,sagging of the printed image would occur. The thickening or gellingagent prevents sagging of the printed image during storage (particularlyvertical storage) and/or heat curing. The addition of the thickening orgelling agent to the protective coating composition of this inventionoperates to improve the screen printing capabilities of the protectivecoating composition by thickening it, to a soft "gel-like" state.

This soft "gel-like" state can be described as being a non-Newtonianfluid which, when free-standing, does not flow. When a force is appliedsuch as by a screen-printing squeegee, the flow occurs, and when suchapplied force is removed, the composition returns to its non-flowing,non-sagging state.

It has been found that when the alicyclic polyamines are employed bythemselves as a curing agent in the protective coating compositions ofthis invention, resistance to flux removal processes may suffer.Examples of such flux removal processes are methylene chloride vapordegreasing and hot water scrubbing or brushing using machines.

In order to regain resistance to flux removal processes such as inmethylene chloride vapor degreasing, it has been found that an aromaticamine should be included as a secondary curing agent in the curing agentcomposition. Preferred aromatic amines include tris-2-ethyl hexoate saltof tris(dimethylaminomethyl)phenol; diamino diphenyl sulfone; benzyldimethyl amine; metaphenylene diamine; and methylene dianiline. This isbecause the aromatic amine cured epoxy resin system gives more rigid,heat and solvent resistant structures, as is well known.

Since the alicyclic polyamines are a strong base, the desired gelstructure or thixotropic character necessary for smearless screenprinting processes and which prevents sagging during storage in avertical orientation also is lost about 5 minutes after the alicyclicpolyamine is added to an epoxy resin solder mask composition,particularly when silica aerogels are used as thickeners.

To reduce or eliminate the basicity of the alicyclic amine in theprotective coating composition of this invention, while retaining thethixotropic character of such protective coating composition, a numberof preselected organic acids should also be included in the curing agentcomposition. These organic acids may be included in the resin system,the curing agent, or in the combination of the resin system and curingagent. Preferred organic acids include (a) carboxy terminatedbutadiene-acrylonitrile polymers and carboxy terminated butadienepolymers; (b) fatty acids such as linoleic acid, oleic acid and thelike; an example of such a fatty acid is a dimer or trimer fatty acidwith two or three carboxylic groups per molecule produced by thepolymerization of C₁₈ fatty acids such as (1) Empol 1040, asubstantially trimer fatty acid (commercially available from EmeryIndustries, Inc., Cincinnati, Ohio 45202) having an acid value of175-192, a saponification value of 192-200, and being 20% dimer acid and80% trimer acid (2) Dimac S, a dimer acid (commercially available fromVictor Wolf Limited, Manchester, England) having an acid number of180-190, a saponification value of 192-198 and containing 8-10% monomer,65-69% dimer, the balance, 26-30%, being higher polymers; and (c)dibasic acids such as adipic acid, glutaric acid, azelaic acid, sebasicacid and suberic acid.

Carboxyl terminated liqid acrylonitrile rubbers with or without pendantcarboxyl groups form an amine salt with the basic alicyclic polymers.The reaction to the salt proceeds smoothly and quickly in a manner ofminutes without external energy and with a slight, but noticeableexotherm occurring. The main benefit is maintaining gel. Other importantbenefits of using carboxyl rubbers are: (1) they provide toughening byreacting with the highly functional epoxy resin and (2) they eliminateor retard carbonate formation. Alicyclic polyamines such as methanediamine will, by themselves, on prolonged standing in air, absorb carbondioxide from the air to form a useless white carbonate salt.

In preparing a permanent protective coating according to the presentinvention, an amount of curing agent is employed which is sufficient tocompletely cure the highly functional epoxy resin in the protectivecoating. This may be calculated according to the amine equivalent weight(A.E.W.) of the curing agent and the epoxy equivalent weight (E.E.W.) ofthe epoxy resin according to the formula: ##EQU1##

Various modifiers are added to the epoxy resin-solvent solution toimprove flow, screenability, to increase toughness and optionally toimpart color, antioxidation and antisag properties.

Flow promoters (or flow control agents) prevent "fish eyes" from formingin a screening mask by lowering surface tension to provide a smooth,continuous surface. As indicated in N. I. Gaynes et al., Formulation ofOrganic Coatings (D. Van Vostrand Co. 1967) pp. 184-185, 294-295 and300-301, resin films tend at times to crater or pinhole. A flow promoterhelps to provide a smooth, homogeneous film, without the unsightlycraters and "orange peel" effect normally developed with the use ofthese resins. Examples of suitable flow promoters include alkyl acrylatepolymers and silicones. Preferred commercial flow promoters are MODAFLOW(commercially available from Monsanto Chemical Co., St. Louis, Mo.);RAYBO 15 (commercially available from Raybo Chemical Co., W. Va.), andDC 840 (commercially available from Dow Corning Co., Midlands, Mich.).MODAFLOW is a high molecular weight polymer believed to be a blend ofisobutylacrylate and ethyl acrylate polymers or a copolymer thereofwhich is commercially available from the Monsanto Chemical Co., St.Louis, Mo. MODAFLOW enhances the surface leveling properties of thecomposition such that the composition when applied to the insulating orconductive bases flows out to a smooth level without forming ripples orbubbles, so that on drying, it produces a smooth, glossy surface. Theflow promoters comprise up to about 6 weight %, preferably up to about 4weight % and when used, typically comprise more than about 1 weight % ofthe protective coating composition of this invention.

Screening aids act as a lubricant, facilitating screen printing of thehighly functional epoxy resin containing coating compositions of thisinvention to produce smooth coatings. Suitable screening aids includeepoxy resins which are liquid at room temperature. Preferredcommercially available epoxy resins include DER 330, DER 331 and DER 332(all commercially available from Dow Chemical Co., Midlands, Mich.);EPON 820 (commercially available from Shell Oil Company, New York,N.Y.); EPOTUF 37-151, EPOTUF 37-135, EPOTUF 37-250 (all commerciallyavailable from Reichhold Chemical Co., White Plains, N.Y.), EPI-REZ 508and EPI-REZ 510 (commercially available from Celanese Corp., New York,N.Y.), and ARALDITE 6005 and ARALDITE 6010 (commercially available fromCIBA-GEIGY CO., Ardsley, N.Y.). The screening aids comprise up to about60 weight %, preferably up to about 40 weight % and when used, typicallycomprise more than about 1 weight %, preferably more than about 2 weight% of the protective coating composition of this invention.

Suitable tougheners include the liquid acrylonitrile butadiene copolymerrubbers and solid epoxy resins with high epoxy equivalent weights. Therubber tougheners precipitate out during curing as, for example,rubber-rich microglobules uniformly dispersed throughout the curedpolymer. The microglobules stop crack propagation from occurring in thecured polymer. Preferred rubber tougheners include the followingcommercially available liquid, monomeric, reactive rubbers: CTB, CTBN,CTBNX and ATBN (commercially available from B. F. Goodrich Chem. Co.,Cleveland, Ohio). The solid epoxy resin tougheners toughen theprotective coating composition by flexibilizing the cross-linked rigidstructure. Preferred epoxy resin tougheners include the following epoxyresins listed in Table II which are solid at room temperature with epoxyequivalent weights greater than 350.

                  TABLE II                                                        ______________________________________                                        Toughener                                                                     Resin    EEW       Softening Point                                                                           Manufacturer                                   ______________________________________                                        DER 661  475-575   70-80° C.                                                                          Dow Chemical Co.                               DER 667  1600-2000 113-123° C.                                                                        Dow Chemical Co.                               Epon 1001                                                                              450-550   65-74° C.                                                                          Shell Chemical Co.                             Araldite 7097                                                                          1650-2000 113-123° C.                                                                        Ciba-Geigy Corp.                               ______________________________________                                    

The tougheners comprise up to about 10 weight %, preferably up to about7 weight % and when used, comprise more than about 1 weight % of theprotective coating composition of this invention.

Suitable color agents (pigments) include the following commerciallyavailable pigments: Cyan Green B-15-3100 (Blue Undertone), Cyan GreenY-15-3040 (Yellow Undertone), and Titanium dioxide (Rutile) OR-600 (allcommercially available from American Cyanamid, Wayne, N.J. 07470);Irgazin Yellow 2GLT (commercially available from Ciba-Geigy Corp.,Ardlsey, N.Y. 10501); and Monastral Red RT-79D and Blue BT 417(commercially available from E. I. duPont de Nemours, Wilmington, Del.19899).

Antioxidants can optionally be included in the protective coatingcomposition of this invention as a modifying agent when continuous useof the cured coating at temperatures above about 100° C. iscontemplated. The antioxidants prevent excessive air oxidation of thecoating composition in such usage. Excessive air oxidation wouldotherwise cause the coating composition to become discolored.Brittleness and loss of adhesion to a substrate to which it is coatedare also avoided.

Suitable antioxidants include thioesters such as dialkylthiodipropionate, dilauryl thiodipropionate, distearyl thiodipropionate,and dimyristyl thiodipropionate; phosphites such astris(nonylphenol)phosphite and alkaryl phosphite; and phenolics such asfatty acid modified substituted phenolics, phosphited hindered phenolicsand high molecular weight hindered phenolics (e.g. butylated hydroxytoluene); and mixtures thereof. Preferred antioxidants include acombination of a thioester with a hindered phenolic in a ratio by weightof 9 to 1 hindered phenolic to thioester. The antioxidants comprise morethan about 1 weight %, preferably more than about 3 weight %, less thanabout 10 weight %, preferably less than about 7 weight % of theprotective coating composition of this invention.

Suitable thickening or gelling agents include a fumed silica havingparticles of submicroscopic size with a total surface area of 200 to 400square meters per gram such as Cab-O-Sil (commercially available fromCabot Corporation, Boston, Mass.); organic modified montmorilloniteclays such as Bentone 27, a trialkylaryl ammonium smectite and Bentone38, a tetraalkyl ammonium smectite, an amine treated bentonite (bothBentones are commercially available from NL Industries, 1230 SixthAvenue, New York, N.Y.); a colloidal silica made according to thedisclosures of U.S. Pat. No. 2,574,902 and 2,577,485 such as Ludox(commercially available from E. I. duPont de Nemours & Co., Wilmington,Del.); a silica aerogel having particles of small microscopic size witha total-surface area of 280 m² /g such as SANTOCEL Z (commerciallyavailable from Monsanto Chemical Co., St. Louis, Mo.).

The thickening or gelling agent comprises up to about 5 weight % andwhen employed, typically comprises more than about 1 weight %,preferably more than about 2 weight % of the epoxy resin solids contentof the protective coating composition of this invention. Upon curing ofthe protective coating composition containing the thickening or gellingagent, the "gel-like" composition remains in its soft "gel-like state"until it is fully cured and solidified in situ without bleed, flow orsag occurring.

Fumed silica may also be added to the primary curing agent to provide itwith body and pourability with less splashing. The advantage of suchadmixture is the minimization of potential weighing errors which mayoccur when the primary curing agent composition is added to theprotective coating composition of this invention.

The apparent viscosity of the epoxy resin screen printable mask isbetween about 10,000 centipoise and about 200,000 centipoise, preferablybetween about 15,000 and about 100,000 centipoise. For purposes ofcontrol, the apparent viscosity of the gels of this invention aremeasured with the Brookfield Viscometer at 10 RPM with a No. 7 spindle.

As will be clear from the following description, use of the permanentprotective coatings of the present invention eliminates bleeding uponcure of the coatings and therefore eliminates the problems concomitantwith the use of resinous masking compositions which bleed or spread whencured. Also important is the fact that the use of these permanentprotective coatings leads to the achievement of high resolution at theedges of the coatings, a result not possible with conventional maskingcompositions. Consequently, improved high density printed circuit boardsmay be produced by various methods of producing circuit boards whichinvolve the use of the improved permanent protective coatings of thisinvention.

Other objects and advantages of the invention will be set forth in partherein and in part will be obvious herefrom or may be learned bypractice with the invention, the same being realized and attained bymeans of the instrumentalities and combinations pointed out in theappended claims.

The invention is more fully described hereinafter with reference to theaccompanying drawings which illustrate certain embodiments of theinvention and together with the specification serve to explain theprinciples of the invention.

FIGS. 1-10 illustrate procedures which can be used to produce printedcircuit boards from insulating substrates and for alternativeembodiments of printed circuit boards produced in accordance with theteachings herein; and

FIGS. 11-13 are flow sheets illustrating typical procedures for makingprinted circuits following the teachings of this invention.

In the drawings, similar reference numerals are used to representsimilar parts.

A method for making printed circuits using an insulating substrate 10 isillustrated in FIG. 1. As shown in FIG. 1A, the insulating substrate 10has bonded thereto metal layers 12 and 14 on both sides of substrate 10.The metal layers 12 and 14 typically are copper. In FIG. 1B, hole 15 isdrilled through the copper clad substrate 10. The substrate 10 is thenimmersed in a palladium-tin chloride solution to apply a thin film ofcatalyst 18 in hole 16 as shown in FIG. 1C. Copper is then electrolesslydeposited, by methods well known in the art, through the hole 16 to forma copper conductive film 20 about 1 micron thick on the walls of thehole 16, as shown in FIG. 1D. In FIG. 1E, there is shown a temporarymask 22 comprised of an alkaline strippable resist such as ER 3057resist (commercially available from Colonial Printing, Inc. Company, 180East Union Avenue, Rutherford, N.J. 07073) which has been screen printedin a negtive pattern. Copper 24 is electroplated and then solder 26 isplated onto the exposed hole 16 and conductor 21 (FIG. 1F). Thetemporary mask 22 is then stripped with a hot alkaline solvent and thecopper layers 12 and 14 are etched with ammonium persulfate or ammonicalcopper chloride solution (FIG. 1G). A permanent solder mask 30 accordingto the present invention may be then applied over the circuit lines,leaving hole 16 exposed (FIG. 1H).

Alternatively, to remove the solder from underneath the solder mask, thesolder can be stripped from the board with hydrogen peroxide andhydrochloric acid solution before applying the solder mask.

In order to preserve adhesion of the mask of this invention to copper,diamines, aromatic diamines such as p-phenylene diamine, aryl amines,hindered phenols such as butylated hydroxy toluene, and thioesters suchas dialkyl thio-dipropionates may be employed.

A method for making printed circuits using a catalytic insulatingsubstrate is illustrated in FIG. 2. As shown in FIG. 2A, a catalyticinsulating substrate 100 has bonded thereto metal layers 102 and 104. Inits simplest form, the substrate 100 has distributed therein an agent(not shown) which is catalytic to the reception of electroless metalfrom an electroless metal deposition solution. Hereinafter, whenever theterm "catalytic" is employed it will refer to a material which has thisproperty, i.e., the ability to receive a metal deposit when exposed toan electroless metal deposition solution, or to catalyze the depositionof metal from such a solution. The catalytic agent may be dissolved inor dispersed throughout the substrate 100. Alternatively, the insulatingbase metal, e.g., the insulating base material may be formed in whole orin part of an insulating organometallic compound which is catalytic tothe reception of electroless metal. Superimposed on the substrate 100and adhered thereto are thin unitary and integral metal layers 102 and104 which preferably cover and are substantially conterminous with,i.e., each have the same boundaries as, the surface of substrate 100.The thickness of the metal layers 102 and 104 will depend primarily uponthe manner in which they are fabricated and bonded to the substrate 100,and will also depend upon the ultimate use to which the substrate is tobe put. Typically, the metal layers 102 and 104 will have a thickness ofbetween about 0.05 micron and 105 microns. In a preferred embodiment,the metal layers 102 and 104 are copper. The thickness of the metallayers 102 and 104 when made of copper will preferably be such thattheir weight will vary between about 0.03 and 2 ounces per square foot.

When the metal layers 102 and 104 are superimposed on the substrate 100by means of conventional metal cladding techniques, i.e., by preforminga thin foil of metal, e.g., by electrolytic deposition, and laminatingit to the base, the foils 102 and 104 will each have a thickness of atleast about 8 microns. On the other hand, if the metal layers areproduced by vapor deposition or by the electroless chemical metaldeposition technique described herein, they can be as thin as 0.05micron.

Using a print and etch technique of the type described hereinbelow,circuit 106 is imposed on the catalytic substrate 100. In FIG. 2B, apositive pattern of the desired circuit is made on the surface of thesubstrate 100 by printing a positive pattern of the desired circuit oneach surface by means of a screen stencil step and repeat negative 116with an acid resistent material 115, such as In FIG. 2C, the metal onboth surfaces in the area not covered by the mask is ready to be etchedto remove the metal foil 102, 104 and form a conductor pattern 106 shownin FIG. 2D. Following etching, the resist 115 is removed to leaveconductor pattern 106 adhered to substrate 100 as shown in FIG. 2E. Theprinted pattern may be formed on the metal clad substrate 100 in avariety of ways.

In screen printing, the step and repeat negative is used to produce astencil on the silk or wire mesh of the screen frame. The stencil ismade photographically from a negative and reproduces it exactly.

It will be understood that either a positive or a negative image of thedesired conducting patterns may be imposed on the substrate, withsuitable modifications to insure that the final conductive patterndesired is ultimately obtained.

When screen stencil printing is employed, the ink used in printing mustbe acid resistant, so that the portions of the metal foil coveredthereby are not affected by the etching solution when the plate iscontacted therewith.

One etching solution commonly used with copper clad stock is ferricchloride. The etching operation is carried out by either blasting thesurface of the panel with a fine spray of ferric chloride or immersingthe printed sheets, which are held in a rack or on a conveyor, in anagitated tank of ferric chloride. The etching operation is controlled bythe concentration of the etching solution and time of contact, and thesevariables must be carefully controlled empirically for good results.After etching, a water rinsing process is employed to remove all etchingchemicals, thereby preventing contamination of the surface or edges ofthe panel.

Frequently, a bare copper foil circuit is not adequate. If, for example,the circuit pattern is to be used as a switch, slip ring, or commutator,it may be necessary to plate the circuit pattern with silver, nickel,rhodium, gold and similar highly wear resistant metals. Where it isnecessary to solder lugs or other hardware to the pattern, it may beadvisable to have the conductor pattern solder plated. In FIG. 2F thecircuit 106 is coated with a permanent solder mask 108 according to thisinvention to leave exposed lands 107 defining interconnecting pointsbetween the circuits. In FIG. 2G, holes 112 are provided in the lands107. The circuit board as it looks in FIG. 2G is then exposed to anelectroless metal deposition solution of the type described herein todeposit electroless metal 116 on the walls surrounding the holes asshown in FIG. 2H. In an alternative embodiment, the solder mask shown inFIG. 2G, could cover the entire circuit pattern with the exception ofthe holes per se. When the resulting base is exposed to electrolessmetal, the walls of the holes would be plated but there would be nolands exposed. The embodiment of FIG. 2, including the alternativesuggested, represents a preferred commercial process for practicing theinvention described herein. FIG. 2 described supra illustrates animportant embodiment of the invention which can be used to goodadvantage in the production of high density printed circuit boards.

When the circuit density is high, it is extremely difficult to print aregistered solder mask so as to leave the lands surrounding holesexposed, for the simple reason that little free space not covered byconductor lines or not taken up by holes is available on the surface ofthe board. For this reason, holes constitute a serious limiting factoron the density of printed circuit boards made conventionally. Even whengreat precautions are taken in printing the solder mask on high densitycircuit boards of the type described, there is a good possibility of themasks breaking down in part, thereby causing the solder to bridge fromone land to another, which in turn results in short circuiting of thefinished board, or the solder mask may block the holes, therebypreventing proper soldering.

By utilizing the registered mask of this invention shown in FIG. 2, highdensity printed circuit boards are facilitated. As shown in FIG. 2, A-F,there is first produced a printed circuit pattern 106 on a suitableinsulating substrate 100. As described with respect to FIG. 2G, thecircuit pattern may be then covered completely with a solder mask 108with the exception of holes defining cross-over points which extendthrough the mask and the board. As shown in FIG. 2H, the hole is nextmetallized by a suitable technique such as described in U.S. Pat. No.3,269,861 or U.S. Pat. No. 3,259,559 to provide a metal coated wall. Thecompleted circuit board within this embodiment takes the form shown inFIG. 2H. When the circuit board of FIG. 2H is subjected to a solderbath, solder deposits only on the electroless metal deposit 116 on thewalls surrounding holes 112. The mask 108 insures that no solderdeposits on the surface of the circuit board itself. The solder mask ofthis invention substantially eliminates any possibility of solderbridging or mask blocking the holes, regardless of how close the holesor how high the circuit density.

As will be appreciated, this technique permits the holes and lines to beplaced close together, thereby enabling a high density circuit patternto be produced, and also minimizes or eliminates the possibility ofsolder bridging.

A wide variety of additional circuit boards may be produced using thepermanent mask of this invention.

In its simplest form, this technique can be used to provide a one-sidedplated through hole circuit board following the technique illustrated,for example, in FIG. 3. In FIG. 3A is shown a blank which comprises acatalytic substrate 1000 having bonded thereto a thin film of metal1400. In FIG. 3B, a circuit pattern 1402 has been produced on base 1000by following the print and etch principles described hereinabove. InFIG. 3C the lines of the circuit pattern 1402 have been covered with apermanent solder mask 1600 according to the present invention leavingthe pads 1402 exposed. In FIG. 3D holes 1500 are then provided in theboard. The holes can be put in the board before or after forming thepattern 1402 or before or after application of the mask 1600. In FIG.3E, the board has been subjected to an electroless metal solution todeposit a coating of electroless metal 1502 on the walls surrounding theholes 1500. When subjected to a solder bath, the board of FIG. 3E willnot receive solder on the portion of the mask edge 1601 surrounding theholes which is adjacent the surface of the mask. Note, however, that thelower surface 1507 of catalytic substrate may accept on occasionunwanted electroless metal deposits.

To avoid the formation of such deposits on the bottom surface, apermanent resin mask 1600 according to the present invention could besuperimposed on both surfaces of the board to enhance the insulatingcharacteristics of the board. Such one-sided circuit boards would havethe appearance shown in FIG. 4. Here again, the smooth, glossy,protective coating of the mask will prevent unwanted deposits on thelower surfaces surrounding the holes.

The technique for producing two-sided plated through hole boardsutilizing this permanent resin mask concept has already been illustratedin FIG. 2.

It will be appreciated that in the FIG. 2 embodiment, the originalcircuit pattern 106 shown in FIG. 2C could, if desired, be produced bythe additive technique described hereinabove, rather than the print andetch technique. Similarly, in FIGS. 3 and 4, the original circuitpattern 1402 could be produced by the additive technique asdistinguished from the print and etch technique.

In FIG. 5 is shown a way in which multilayer plated through hole boardscould be used utilizing the permanent resin mask concept describedhereinabove. In FIG. 5A is shown a blank comprising a catalyticsubstrate 1000 having superimposed thereon a thin metal film 1200.

In FIG. 5B, a printed circuit pattern 1202 has been formed on thecatalytic substrate 1000 utilizing a print and etch technique.

In FIG. 5C, there has been superimposed upon the circuit pattern 1202 acatalytic blank 1100 comprising a catalytic substrate 1002 and a thinmetal film 1400. A similar catalytic blank 1102 has been superimposed onthe bottom layer of the catalytic substrate 1000 as is also shown inFIG. 5C. In FIG. 5D, additional circuit patterns 1404 have been formedon the top and bottom catalytic substrate layers 1002 using a print andetch technique. As shown in FIG. 5E, the circuit pattern top and bottomis next coated with a permanent solder mask 1600 according to thepresent invention. Next, holes 1500 are provided in the circuit board asshown in FIG. 5F. Finally, the board is subjected to an electrolessmetal deposition solution to deposit electroless metal 1502 on the wallssurrounding holes 1500, as shown in FIG. 5G.

The finished circuit board could then be solder plated, for example, byimmersing in a solder bath to coat the walls of holes 1500 with solder,if desired.

In a further refinement of the embodiment shown in FIG. 5, suitableindicia at 1201 could be superimposed or provided on the substrate 1000,preferably prior to lamination of the metal clad blanks shown broadly at1100 and 1102. These indicia could then be used as bull's eyes forpattern registration during printing of the circuit patterns on metallayers 1400. The bull's eyes 1201 may take a variety of forms. Thus, forexample, they could be holes boarded or otherwise provided in thesubstrate 1000 which could serve as a ground plane or a support for anactual circuit pattern, or both. Alternatively, bull's eyes 1201 couldtake the form of a metal spot or dot produced either by the print andetch or additive technique described herein.

It should also be brought out that the initial circuit pattern 1202 neednot be formed on the ground plane or substrate 1000 by print and etchtechniques. Similarly, the metal clad blanks 1100 and 1102 in FIG. 5could if desired be replaced simply by a catalytic base, following whichthe circuit patterns 1404 could be made by the additive technique ratherthan the print and etch.

In FIG. 6 is shown still a further embodiment for producing multi-layerboards utilizing the registered printed solder mask concept.

In FIG. 6A is shown a blank comprising a catalytic substrate 1000 havingsuperimposed on both surfaces thin metal films 1200.

Using a print and etch technique, a circuit or ground pattern 1202 isformed on the substrate 1000. Here again, if desired, bull's eyes orregistration marks 1201 may be formed on the substrate 1000simultaneously with production of the first printed circuit or groundpattern 1202. In FIG. 6C a catalytic blank 1100 comprising a catalyticsubstrate 1000 and a thin metal film 1400 has been superimposed oncircuit pattern 1202 and laminated to the catalytic substrate 1000.

Using print and etch techniques, a circuit pattern 1404 is formed on thesurface of substrate 1002 and on the lower surface of ground substrate1000 as shown in FIG. 6D. In FIG. 6E, a permanent registered mask 1600according to the prevent invention is superimposed on circuit pattern1404 so that the resulting three-layered board appears as shown in FIG.6E.

In FIG. 6F holes 1500 are provided in the board, following which theboard is subjected to an electroless metal deposition solution todeposit metal on the walls surrounding the holes. The finishedthree-layer circuit board is shown in FIG. 6G, wherein 1501 refers tothe metal surrounding the walls of the holes.

In FIG. 6, circuit pattern 1202 need not be formed by the print and etchtechnique. Rather, it could be formed by the additive process describedherein. Similarly, blank 1100 need not be a metal clad blank but couldsimply consist of a catalytic substrate. In such an embodiment, circuitpattern 1404 could also be produced by the additive technique.

In FIG. 7 is shown a particular important embodiment of the inventionwherein a four-layer plated throughhole circuit board containing aregistered permanent solder mask is produced. In FIG. 7A is shown ablank comprising a substrate 1000 having superimposed thereon thin metalfilms 1200. In FIG. 7B print and etch circuit patterns 1202 are formedon the top and bottom layer of the ground plane 1000. At 7C, blanks 1100and 1102 are laminated top and bottom to the substrate 1000 and coverthe circuit patterns 1202. Blanks 1100 and 1102 comprise a catalyticsubstrate 1002 and a thin metal film 1400.

In FIG. 7D, additional circuit patterns 1402 are formed top and bottomby print and etch techniques. Here again, bull's eyes or registry marks1201 may be used if desired for registration purposes in forming thecircuit patterns 1402. In FIG. 7E a permanent registered solder mask1600 according to the present invention is coated over both the top andbottom surfaces on the board.

Next, holes 1500 defining cross-over points are provided in the boardshown in FIG. 7F. Then the board is subjected to electroless metaldeposition to deposit metal in the walls surrounding the holes is shownin FIG. 7G. Here again, in the FIG. 7 embodiment, it is not necessary toform the circuit patterns 1402 and 1202 by print and etch techniques.Rather, each of these circuit patterns could be produced using theadditive technique described herein. In such embodiments, the metalcladding 1200 and 1400 would be eliminated.

In FIG. 8, 210 is a catalytic substrate and 218 is a catalytic adhesivesuperimposed on the substrate 210. Numeral 117 represents the printedcircuit pattern and 216 represents the permanent solder mask accordingto the present invention. In FIG. 8, the wall of hole 215 has a depositof metal 221 to make a through connection. Note that the plating on thehole wall extends over the edge of the board at the lower surface toform a small exposed eyelet 221 (a).

To avoid the formation of eyelets 225 (a) on the lower surface, thatsurface could be coated with a permanent or nonpermanent mask as shownin FIG. 9.

In FIG. 9 is shown a plated through hole formed utilizing the registeredpermanent resin mask concept. In FIG. 9, the board comprises a catalyticbase 210, the lower surface of which comprises an insulating resinouslayer 211. On the upper surface, the base 210 is coated with a layer 218of catalytic adhesive material on which has been formed the printedcircuit pattern 217. A permanent resin mask 216 according to the presentinvention covers the circuit pattern completely. Hole 215 has its wallscovered with metal 221, which terminates short of the outer surfaces ofmasks 216 and 211.

In FIG. 10 is shown a plated through hole board. The board of FIG. 10comprises a catalytic base 210 having superimposed thereon layers ofcatalytic adhesive 218. Printed circuit pattern 217 is formed on theupper surface of adhesive 218. Permanent solder mask 216 according tothe present invention covers the entire circuit pattern. Hole 215 has aplated wall 221. Here the plated wall extends to the lower surface ofthe board. In FIG. 10, the electroless metal layer 221 will grow overthe lower surface of catalytic ink 218 to form eyelets as shown at 221(a) in FIG. 8 unless this surface is coated with a temporary mask.Accordingly, to produce the board shown in FIG. 10, a temporary maskwill have to be superimposed over the lower surface of catalyticadhesive 218 before electroless metal deposits and then stripped afterdeposition. Alternatively, the mask on the lower surface could bepermanent.

In FIGS. 8 through 10, additional circuit patterns could be imposed onthe lower surface as well as the upper surface of the base following thetechniques described herein.

To place the embodiments of FIGS. 1-10 in proper perspective, it shouldbe pointed out that whenever distance between hole centers in platedthrough hole printed circuit boards is 2.5 mm or less, silk screenprinting techniques break down due to ink bleed, so that when highdensity circuit patterns are desired having hole center spacing of 2.5mm or less, photoprinting techniques will ordinarily be used. It isexactly on such high density boards that conductors between the holescause the difficulties described above. The permanent non-bleedingsolder mask concepts for protecting conductors between plated throughholes described in connection with the identified drawings is thereforeparticularly applicable to manufacture of printed circuit boards havingplated through holes whose centers are spaced apart 2.5 mm or less,i.e., between about 1.2 to 2.5 mm; and particularly between 0.8 and 1.2mm.

It should again be emphasized that the permanent registered insulatingmask concept described herein, although suitable for the manufacture ofa wide variety of printed circuit boards, has exceptional advantageswhen used in combination with the catalytic bases described herein toproduce high density plated through hole printed circuit boards. Such atechnique using the identified materials represents a practical way forachieving even plating on the walls of small, high aspect ratio (smalldiameter with respect to the thickness of the part) holes. Heretofore,using conventional techniques and materials, the plating on the holewalls has tended to be uneven.

The non-bleeding permanent solder mask concept avoids the problemsheretofore described of printing a permanent insulating mask. As hasbeen brought out above, it is very difficult using modern printingconcepts to print a registered permanent solder mask on boards on whichthe hole centers are spaced a distance less than 120 mils. The consensusin the art is that when the holes are less than 100 mils apart, it ispractically impossible to print a registered permanent solder mask.

FIGS. 11-13 are flow sheets of procedures for manufacturing a variety ofprinted circuit products which contain copper as the primary conductivemetal using the materials, compositions and techniques described herein.These flow sheets are self-explanatory when red in the light of theforegoing specification. Of course, the procedures of FIGS. 11-15 areequally applicable to products which use metals other than copper, e.g.,nickel, as the primary conductive metal.

The following examples illustrate at least one of the best modes of theprotective coating compositions and methods of the present invention aspresently understood.

Five typical mask compositions are given hereinbelow:

EXAMPLE 1 Formulation 1: Solder Mask Composition

    ______________________________________                                                                  Amount by                                           Material                  Weight (g)                                          ______________________________________                                        A. Formulation Makeup                                                         50% by weight difunctional bisphenol A epoxy                                                            537                                                 resin having a Durran softening point of                                      about 75° C. and an epoxy equivalent weight of                         475-575 in diethylene glycol ethyl ether                                      solvent (epoxy coating modifier)                                              70% by weight epoxidized novolac diglycidyl                                                             83.4                                                ether bisphenol A resin in diethylene glycol                                  ethyl ether solvent with a 4.4 average                                        epoxide functionality and a melting point of                                  99° C.                                                                 diethylene glycol ethyl ether (solvent)                                                                 29.1                                                25% by weight copper phthalocyanine                                                                     18.3                                                pigment (dispersed in bisphenol A                                             epoxy resin with an epoxide equivalent                                        weight of 180-190 and a viscosity of                                          11,000 to 14,000 at 25° C. as green                                    colorant                                                                      a fumed silica having an average particle                                                               33.3                                                size of 0.015 microns and a total measured                                    surface area of 200 m.sup.2 /g (thickening agent)                             polyisobutyl acrylate (flow promoter)                                                                    4.2                                                a liquid epoxy novolac phenol formaldehyde                                                              18.2                                                resin having an epoxy equivalent weight                                       of 172-179 and a viscosity of 1400-2000                                       centipoise at 25° C. (screening aid)                                   a synethetic silica, dry white powder having                                                            14.5                                                a surface area of 150 m.sup. 2 /g and an average                              particle size of 0.021 microns (flattening agent)                             B. Primary Curing Agent Composition                                           to be Admixed with A                                                          methane diamine            61                                                 2-ethyl hexoic acid salt of                                                                              7.9                                                tris-(dimethylaminomethyl)phenol                                              carboxy terminated acrylonitrile                                                                        16.5                                                butadiene and pendant carboxyl groups                                         having an average molecular weight                                            of 3500, a carboxyl content of                                                2.37% by weight, a functionality of                                           1.85 and having 18% by weight                                                 bound acrylonitrile                                                           a fumed silica having an average particle                                                                10                                                 size of 0.015 microns and a total                                             measured surface area of 200 m.sup.2 /g                                       (thickening agent)                                                            C. Modifying Additives to be Added                                            Optionally to A and B for Flame                                               and/or Oxidation Resistance                                                   tetrabromobixphenol-A (a brominated                                                                     181                                                 phenol) (flame resistant agent)                                               a non-volatile hindered phenol                                                                           2.2                                                (oxidation resistance)                                                        20% by weight dilauryl thiodipropionate                                                                 11.1                                                in ethylene glycol butyl ether acetate                                        (oxidation resistance)                                                        ______________________________________                                    

Formulation 2: Solder Mask Composition

    ______________________________________                                                               Amount by                                              Material               Weight (g)                                             ______________________________________                                        A. Formulation Make-up Same as in                                             Formulation 1 Hereinabove                                                     B. Primary Curing Agent to be                                                 Admixed with A                                                                50% by weight triethylenediamine                                                                     127                                                    in methyl cellosolve                                                          carboxyl terminated acrylonitrile                                                                    16.5                                                   butadiene and pendant carboxy groups                                          having an average molecular weight of                                         3500, a carboxyl content of 2.37%                                             by weight, a functionality of 1.85                                            and having 18% by weight bound                                                acrylonitrile                                                                 2-ethyl hexoic acid salt of                                                                           4.6                                                   tris-(dimethyl-aminomethyl)phenol                                             (secondary curing agent)                                                      a fumed silica having an average                                                                     10                                                     particle size of 0.015 microns and a                                          total measured surface area of                                                200 m.sup.2 /g (thickening agent)                                             ______________________________________                                    

Formulation 3: Solder Mask

A & B are same as in formulation 2 except that 65 grams ofN-(2-aminoethyl)-piperedine are used instead of 127 grams of 50% byweight triethylenediamine in methyl cellulose.

Formulation 4: Solder Mask

Parts A & B are same as in formulation 2 except than 60 grams ofbis(P-aminocyclohexyl)methane are used instead of 127 grams of 50% byweight triethylenediamine in methyl cellulose.

Formulation 5: Solder Mask

    ______________________________________                                                                  Amount by                                           Material                  Weight (g)                                          ______________________________________                                        A. Formulation Make-up Same as in                                             Formulation 1 Hereinabove                                                     B. Primary Curing Agent Composition                                           to be Admixed with A                                                          menthane diamine          47                                                  dimer acid (primarily composed of C.sub.36                                                              13                                                  dibasic acid having a molecular weight                                        of approximately 565)                                                         2-ethyl hexoic acid salt of tris-(dimethyl-                                                              6                                                  aminomethyl)phenol (secondary curing agent)                                   ______________________________________                                    

Electroless nickel baths which may be used are described in Brenner,Metal Finishing, November, 1954, pages 68-76. They comprise aqueoussolutions of a nickel salt, such as nickel chloride, an active chemicalreducing agent for the nickel salt, such as the hypophosphite ion; and acomplexing agent, such as carboxylic acids and salts thereof.

Electroless nickel baths which may be used are described in Brenner,Metal Finishing, November, 1954, pages 68 to 76. They comprise aqueoussolutions of a nickel salt, such as nickel chloride, an active chemicalreducing agent for the nickel salt, such as the hypophosphite ion; and acomplexing agent, such as carboxylic acids and salts thereof.

Electroless gold plating baths which may be used are disclosed in U.S.Pat. No. 3,589,916. They contain a water soluble gold salt, such as goldchloride, a reducing agent for the gold salt, such as the dimethylamineborane, a chelating or complexing agent, such as sodium potassiumtartrate and a stabilizer such as sodium cyanide. The purpose of thecomplexing agent is to maintain the gold in solution as a water solublegold complex. The pH of the bath will be about 13 or between about 10and 14.

A specific example of an electroless copper depositing bath suitable foruse is a bath having the following composition:

    ______________________________________                                        N,N,N.sup.1,N.sup.1 tetrakis (2-hydroxypropyl                                                         18      g/l                                           ethylenediamine                                                               CuSO.sub.4 -5H.sub.2 O  10      g/l                                           Formaldehyde (37% Soln) 4       ml/l                                          Wetting Agent (GAFAC-RE610)                                                                           0.01    g/l                                           (Commercially available from                                                  GAF Corporation) (believed to be a                                            phosphate ester of alkylphenol-                                               polyethylene oxide)                                                           Sodium hydroxide        to desired pH                                                                 (12-13)                                               Sodium cyanide (NaCN)   25      mg/l                                          2-mercapto benzothiazole                                                                              10      mg/l                                          ______________________________________                                    

This bath is preferably operated at a temperature of about 52° C. andwill deposit a coating of ductile electroless copper about 35 micronsthick in about 18 hours.

Utilizing the electroless metal baths of the type described, very thinconducting metal films may be laid down. Ordinarily, the metal filmssuperimposed by electroless metal deposition will range from 2.5 to 100microns in thickness, with metal films having a thickness of even lessthan 2.5 microns a distinct possibility.

What is claimed is:
 1. In a permanent, protective coating compositionuseful in the manufacture of printed circuit boards, said coatingcomposition comprised of a thermosetting epoxy resin and curingcomposition dissolved in a solvent; and a flow promoter; said curingcomposition present in an amount sufficient to effect substantiallycomplete cure of said thermosetting resin, said coating compositionbeing in an initial state as a high viscosity solution and having anapparent viscosity between about 10,000 centipoise and about 200,000centipoise, said coating composition upon curing, being transformed fromsaid initial state into a final state as a solid, the improvement whichcomprises:said thermosetting epoxy resin being a solid at roomtemperature, having a melting point between about 60° C. and about 200°C., and having an epoxide functionality greater than about 3; saidcuring composition including: a primary curing agent comprised of analicyclic polyamine which is corrosive to copper surfaces, the primarycuring agent being present in an amount sufficient to effectsubstantially complete cure of the thermosetting epoxy resin; and asecondary curing agent comprised of an aromatic amine, said secondarycuring agent being present in an amount sufficient to provide said curedcoating composition with resistance to flux removal processes.
 2. Apermanent, protective coating composition for use in the manufacture ofprinted circuit boards, said composition comprising:a thermosettingepoxy resin dissolved in a solvent, said thermosetting epoxy resin beinga solid at a temperature of 60° C. or lower, having a melting pointbetween about 60° C. and about 200° C., and having an epoxidefunctionality greater than about 3; a primary curing agent comprised ofan alicyclic polyamine which is corrosive to copper surfaces, saidprimary curing agent being present in an amount sufficient to effectsubstantially complete cure of said coating composition, said coatingcomposition being in an initial state as a high viscosity solution andhaving an apparent viscosity between about 10,000 centipoise and about200,000 centipoise, does not liquify during cure, and upon being curedtransforming from said initial state into a final state as a solid; anda secondary curing agent comprised of an aromatic amine, said secondarycuring agent being present in an amount sufficient to provide said curedcoating composition with resistance to flux removal processes.
 3. Acomposition as defined in claim 2 further including a flow promoter andat least one modifying agent selected from the group consisting of ascreening aid and a toughener.
 4. A composition as defined in claim 1wherein said viscosity is between about 15,000 centipose and about100,000 centipoise,
 5. A composition as defined in claim 1 wherein saidprotective coating composition further includes an antioxidant modifyingagent.
 6. A composition as defined in claim 1 wherein said coatingcomposition further includes an antioxidant selected from the groupconsisting of thioesters, phosphites, phenols and mixtures thereof.
 7. Acomposition as defined in claim 1 wherein said thermosetting epoxy resinepoxide functionality is more than about 3 and less than about
 10. 8. Acomposition as defined in claim 1 wherein said thermosetting epoxy resinepoxide functionality is more than about 4 and less than about
 7. 9. Acomposition as defined in claim 3 wherein one of said modifying agentsin said screening aid, said screening aid comprising an epoxy resinwhich is liquid at room temperature, said liquid epoxy resin having anepoxy equivalent weight below about
 225. 10. A composition as defined inclaim 3 wherein said flow promoter comprises an alkyl acrylate polymeror a silicone.
 11. A composition as defined in claim 3 wherein one ofsaid modifying agents is said toughener, said toughener comprising aliquid acrylonitrile butadiene copolymer, a solid epoxy resin with anepoxy equivalent weight about 350 or a combination thereof.
 12. Acomposition as defined in claim 1 where said alicyclic polyamine isselected from the group consisting of methane diamine;1,3-diamino-cyclohexane; isophorone diamine; triethylenediamine; andhexamethylenetetramine.
 13. A composition as defined in claim 1 wheresaid aromatic amine is selected from the group consisting oftris-2-ethyl hexoate salt of tris(dimethylaminomethyl)phenol; diaminodiphenyl sulfone; benzyl dimethyl amine; metaphenylene diamine; andmethylene dianiline.
 14. A permanent, protective coating composition foruse in the manufacture of printed circuit boards, said compositioncomprising:an epoxy resin dissolved in a solvent, said epoxy being asolid at room temperature, having an epoxide functionality more thanabout 3 and less than about 10, and having a melting point between about60° C. and about 200° C.; a screening aid comprised of an epoxy resinwhich is liquid at room temperature; a flow promoter comprised ofacrylate polymers and/or silicone; a toughener comprised of a liquidacrylonitrile polybutadiene and/or a solid epoxy resin having an epoxyequivalent weight above about 350; a primary curing agent comprised ofan alicyclic polyamine which is corrosive to copper surfaces, saidcuring agent being present in an amount sufficient to effectsubstantially complete cure of said coating composition, said coatingcomposition upon being cured transforming from a high viscosity solutionhaving an apparent viscosity between about 10,000 centipoise and about200,000 centipoise into a solid; and a secondary curing agent comprisedof an aromatic amine, said secondary agent being present in an amountsufficient to provide said cured coating composition with resistance toflux removal processes.
 15. A composition as defined in claim 14 furtherincluding an antioxidant comprised of a thioester, phosphite, phenol ora mixture thereof.
 16. A composition as defined in claim 2, wherein saidviscosity is between about 15,000 centipoise and about 100,000centipoise.
 17. A composition as defined in claim 2 wherein saidprotective coating composition further includes an antioxidant modifyingagent.
 18. A composition as defined in claim 2 wherein said coatingcomposition further includes an antioxidant selected from the groupconsisting of thioesters, phosphites, phenols and mixtures thereof. 19.A composition as defined in claim 2 wherein said thermosetting epoxyresin epoxide functionality is more than about 3 and less than about 10.20. A composition as defined in claim 2 wherein said thermosetting epoxyresin epoxide functionality is more than about 4 and less than about 7.21. A composition as defined in claim 2 where said alicyclic polyamineis selected from the group consisting of methane diamine;1,3-diamino-cyclohexane, isophorone diamine; triethylenediamine; andhexamethylenetetramine.
 22. A composition as defined in claim 2 wheresaid aromatic amine is selected from the group consisting oftris-2-ethyl hexoate salt of tris(dimethylaminomethyl)phenol; diaminodiphenyl sulfone; benzyl dimethyl amine; metaphenylene diamine; andmethylene dianiline.